Background: The observed low metastatic potential and favorable survival of small incidentally detected renal cell carcinomas (RCCs) have been a part of the rationale for recommending partial nephrectomy as a first treatment option and active surveillance in selected patients. We examined the relationship between tumor size and the odds of synchronous metastases (SMs) (primary outcome) and disease specific survival (secondary outcome) in a nationwide RCC registry. Methods: Retrospective study of the 794 RCC patients diagnosed in Iceland between 1971 and 2005. Histological material and TNM staging were reviewed centrally. The presence of SM and survival were recorded. Cubic spline analysis was used to assess relationship between tumor size and probability of SM. Univariate and multivariate statistics were used to estimate prognostic factors for SM and survival. Results: The probability of SM increased in a non-linear fashion with increasing tumor size (11, 25, 35, and 50%) for patients with tumors of ≤4, 4.1-7.0, 7.1-10.0, and >10 cm, respectively. On multivariate analysis, tumor size was an independent prognostic factor for disease-specific survival (HR = 1.05, 95% CI 1.02-1.09, p < 0.001), but not for SM. Conclusion: Tumor size affected the probability of disease-specific mortality but not SM, after correcting for TNM staging in multivariate analysis. This confirms the prognostic ability of the 2010 TNM staging system for renal cell cancer in the Icelandic population.